horizon mapping in cycles
This didn't work because the Texture Coordinates node wasnt added. You should plug one of its output sockets (e.g. Generated or Object) into the Vector input socket of the Mapping node (this is a common approach if texturing anything in Cycles. Otherwise, in Cycles, texture won't appear on the mesh, just solid base color). 



Thus you'll be able to adjust the position of the horizon simply by changing Location value by Z axis of the Mapping node (not sure if this is good idea to manipulate horizon like that).



(I changed Location value to 0.3 here).